1. Field of the Invention
The present invention relates to a method of producing cast coated paper, and more particularly, to the method in which the near-infrared radiation serves as heating means in the step of heating a layer of wet coating applied on a web of paper to permit a gel structure to be formed therein, or to permit the wet layer of coating to be dried.
2. Description of the Prior Art
As is well known, there are several conventional methods of producing cast coated high-gloss paper for printing. These methods include (1) a wet casting method adapted to produce a glazed coated layer on a web of paper by applying a layer of coating comprising a mineral pigment and an adhesive to a web of paper and pressing the wet coating layer against the heated highly polished surface of a casting drum or roll, (2) a rewet casting method in which the applied layer of wet coating is first dried, then plasticized with a rewetting liquid and pressed against the heated polished surface of a casting drum, and (3) a gel-casting method adapted to produce a glazed coated layer by pressing the coating layer in a gel state against a heated polished surface of a casting drum.
In any of these conventional casting methods, the coating layer in a water plasticized state is pressed against the highly polished finishing surface of the heated drum by means of a press roll so as to be dried in contact therewith and released from the heated drum thereby producing a glazed finish on the surface of the coated layer.
Of these conventional methods, in the rewet casting method, and the gel casting method, the coating layer is in a dried state or having a gel state before the layer is pressed against the finishing surface of the heated drum. Therefore, the coating layer can be pressed against the drum having a surface temperature of at least 90.degree. C., and the cast coating operation can be performed at a speed much higher than that of the wet casting method, without any fear of rupturing the coating layer or of breaking the paper, both of which result from a rapid evaporation of a large amount of moisture in the coating layer in the wet casting method.
However, in the rewet casting method, since the coating layer, having been dried,must be rewetted, plasticization of the coating layer is liable to be insufficient. On the other hand, in the gel-casting method, when a sufficiently firm gel state is not formed in the coating layer, the coating layer tends to be pressed against the drum in a poor manner causing an uneven and loose contact with the drum. However, in these cases, it is possible to produce a cast coated paper having a relatively uniform glazed finish when the cast coating operation is conducted at a relatively low speed. It is difficult to produce the cast coated paper having such relatively uniform glazed finish when the cast coating operation is conducted at a relatively high speed, because many defects such as pinhole-like dots, gloss irregularity and the like are involved in such high-speed operation. Furthermore, in each of the rewet casting method and the gel-casting method, a so-called "mottling" problem, which is a gloss irregularity appearing in the surface of the cast coated paper, is most likely to occur. Hitherto, in order to resolve the above problems, pigments excellent in air-permeability have been employed, while the base paper has been improved in its drying characteristics. However, a satisfactory result is still not obtained.
Hitherto, in both of the rewet casting method and the gel-casting method, there have been employed suitable heating/drying means for the wet coating layer, such as steam heaters, microwave, laser, electron-beam and the like. Among these heaters, the gas heaters and the electrical heaters, which are generally well-known as a drying device, emit middle-or-far-infrared radiations having wave lengths longer than 2.5 .mu.m at the peak wave length of their ranges or other infrared radiations but do not emit near-infrared radiations. In case that the infrared radiations except the near-infrared ratiations are utilized for drying the wet coating layer, prior to casting or utilized for evaporating the moisture of the layer to permit a gel structure to be formed in the layer, only a surface portion of the layer is rapidly dried so that the infrared radiations, other than the near-infrared radiations, are not adequate to dry the wet coated layer in the casting methods. Consequently, as will be clarified later, it is impossible for middle-or far-infrared radiations to realize predetermined effects of the present invention.